s active in both insect and other animals. Therefore, B. thuringiensis strains which produce ¯-exotoxin are not generally used for commercial bioinsecticide production (2:18).
In contrast, the commercially important (-endotoxin comprises a large proportion of the B. thuringiensis parasporal crystal. Angus (1954) established the crystals' toxicity for lepidopteran larvae (caterpillars). Moreover, in 1959, Heimpel and Angus classified the larvae according to their toxin susceptibility. For example, their "type I" insects are killed by (-alone; whereas, in their "type II" category, the effect of (-endotoxin is enhanced by the presence of spores. Lastly, Heimpel and Angus (1959) "type III" insects are only killed by combined spore and endotoxin mixtures. For the most part, susceptible insects tend to fall into the type I category. Although insect midgut alkalinity is necessary for crystal dissolution, in some instances the pH is too high for spore germination. Subsequent endotoxin activity, however, may actually decrease the midgu
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